The invention relates to methods and compositions for making a multi-layer article.
Multi-layer articles can be used in a variety of applications. For example, superconductors, including oxide superconductors, can be formed of multi-layer articles. Typically, such superconductors include a layer of superconductor material and a layer, commonly referred to as a substrate, that can enhance the mechanical strength of the multilayer article.
Generally, in addition to enhancing the strength of the multi-layer superconductor, the substrate should exhibit certain other properties. For example, the substrate should have a low Curie temperature so that the substrate is not ferromagnetic at the superconductor""s application temperature. Furthermore, chemical species within the substrate should not be able to diffuse into the layer of superconductor material, and the coefficient of thermal expansion of the substrate should be about the same as the superconductor material. Moreover, if the substrate is used for an oxide superconductor, the substrate material should be relatively resistant to oxidation.
For some materials, such as yttrium-barium-copper-oxide (YBCO), the ability of the material to provide high transport current in its superconducting state depends upon the crystallographic orientation of the material. For example, such a material can exhibit a relatively high critical current density (Jc) when the surface of the material is biaxially textured.
As used herein, xe2x80x9cbiaxially texturedxe2x80x9d refers to a surface for which the crystal grains are in close alignment with a direction in the plane of the surface. One type of biaxially textured surface is a cube textured surface, in which the crystal grains are also in close alignment with a direction perpendicular to the surface. Examples of cube textured surfaces include the (100)[001] and (100)[011] surfaces, and an example of a biaxially textured surface is the (113)[211] surface.
For certain multi-layer superconductors, the layer of superconductor material is an epitaxial layer. As used herein, xe2x80x9cepitaxial layerxe2x80x9d refers to a layer of material whose crystallographic orientation is directly related to the crystallographic orientation of the surface of a layer of material onto which the epitaxial layer is deposited. For example, for a multi-layer superconductor having an epitaxial layer of superconductor material deposited onto a substrate, the crystallographic orientation of the layer of superconductor material is directly related to the crystallographic orientation of the substrate. Thus, in addition to the above-discussed properties of a substrate, it can be also desirable for a substrate to have a biaxially textured surface or a cube textured surface.
Some substrates do not readily exhibit all the above-noted features, so one or more intermediate layers, commonly referred to as buffer layers, can be disposed between the substrate and the superconductor layer. The buffer layer(s) can be more resistant to oxidation than the substrate, and reduce the diffusion of chemical species between the substrate and the superconductor layer. Moreover, the buffer layer(s) can have a coefficient of thermal expansion that is well matched with the superconductor material.
Typically, a buffer layer is an epitaxial layer, so its crystallographic orientation is directly related to the crystallographic orientation of the surface onto which the buffer layer is deposited. For example, in a multi-layer superconductor having a substrate, an epitaxial buffer layer and an epitaxial layer of superconductor material, the crystallographic orientation of the surface of the buffer layer is directly related to the crystallographic orientation of the surface of the substrate, and the crystallographic orientation of the layer of superconductor material is directly related to the crystallographic orientation of the surface of the buffer layer. Therefore, the superconducting properties exhibited by a multi-layer superconductor having a buffer layer can depend upon the crystallographic orientation of the buffer layer surface.
The invention relates to methods and compositions (e.g., precursor solutions) for making a multi-layer article, such as a multi-layer superconductor. In part, the methods are based upon the recognition that the physical characteristics of a superconductor material (e.g., an oxide superconductor material) formed using a precursor solution containing a trifluoroacetate salt of one or more metals can depend upon the total water content (e.g., liquid water in the solution and water vapor in the surrounding environment) present when treating the precursor solution to form an intermediate of the superconductor material. Thus, by properly balancing the total water content, as well as certain other conditions (e.g., temperature, temperature ramp rate and oxygen pressure), a good quality superconductor material layer can be prepared. In certain embodiments, the superconductor material can be prepared in a relatively short period of time. The methods can involve the use of a precursor solution having a relatively low water content and/or a relatively high solids content. The methods can also involve treating the precursor solution in an environment having a relatively high vapor pressure of water.
In one aspect, the invention features a method which includes disposing a solution on the surface of a first layer. The solution contains a salt of a first metal, a salt of a second metal and a salt of a rare earth metal. At least one of the metal salts is a trifluoroacetate salt. The method also includes treating the solution for less than about five hours to form a layer of an intermediate of a rare earth metal-second metal-first metal-oxide. The layer of the intermediate has a surface adjacent the surface of the first layer. Defects contained within the layer of the intermediate make up less than about 20 percent of any volume element of the layer of the intermediate defined by the projection of one square centimeter of the surface of the layer of the intermediate, and the layer of the intermediate is free of any defect having a maximum dimension of greater than about 200 microns.
As used herein, a xe2x80x9cdefectxe2x80x9d refers to a crack or a blister, such as a crack or blister that is detectable by visual (or optical) inspection.
A volume element of a layer of material defined by the projection of a given area of a surface of the layer of material corresponds to the volume of the layer of material whose edges are perpendicular to the given area of the surface of the layer of material.
The intermediate can be, for example, partially or completely formed of one or more metal oxyfluoride compounds.
In another aspect, the invention features a method which includes disposing a solution on the surface of a first layer. The solution includes water, a first salt of a first metal, a second salt of a second metal and a third salt of a rare earth metal. At least one of the first, second and third salts being a trifluoroacetate, and the solution has a water content of less than about 50 volume percent. The method also includes treating the solution to form an intermediate of a rare earth metal-second metal-first metal-oxide.
In a further aspect, the invention features a method which includes disposing a solution on the surface of a first layer. The solution contains a salt of a first metal, a salt of a second metal and a salt of a rare earth metal. At least one of the metal salts is a trifluoroacetate salt. The method also includes treating the solution for less than about five hours to form an intermediate of a rare earth metal-second metal-first metal-oxide. The intermediate is capable of undergoing further processing to provide a superconductor material having a critical current density of at least about 5xc3x97105 Amperes per square centimeter (e.g., at least about 1xc3x97106 Amperes per square centimeter, at least about 1.5xc3x97106 Amperes per square centimeter, or at least about 2xc3x97105 Amperes per square centimeter).
The first metal can be copper, and the second metal can be barium, strontium or calcium. The rare earth metal can be yttrium. For example, the rare earth metal-first metal second metal-oxide can be a yttrium-barium-copper-oxide, such as YBa2Cu3O7xe2x88x92x.
Treating the solution can include heating the solution from an initial temperature (e.g., room temperature) to a temperature of from about 190xc2x0 C. to about 215xc2x0 C. (e.g., about 210xc2x0 C.) at a rate of about at least about 5xc2x0 C. per minute then heating to a temperature of from about 220xc2x0 C. to about 290xc2x0 C. (e.g., about 220xc2x0 C.) at a rate of from about 0.05xc2x0 C. per minute to about 0.4xc2x0 C. followed by heating to about 400xc2x0 C. at a rate of at least about 2xc2x0 C. per minute to form an intermediate. The heating steps can be performed in a nominal gas environment having a water vapor pressure of about from about 5 Torr to about 50 Torr. The nominal gas environment can have an oxygen pressure of from about 0.001 Torr to about 760 Torr.
The intermediate can be further heated to form the rare earth metal-second metal-first metal-oxide, which can have a relatively high critical current density (e.g., at least about 1xc3x97106 Amperes per square centimeter). The rare earth metal-second metal-first metal-oxide layer can be biaxially textured.
The first layer can be a substrate, a buffer layer, or a superconductor layer, and the surface of the first layer can be biaxially textured.
One potential advantage of the invention is that it can provide methods and compositions for forming high quality superconductor layers or layers of superconductor intermediate (e.g., metal oxyfluoride intermediate) in a relatively short period of time. For example, the methods and compositions can be used to prepare a superconductor intermediate in less than about five hours (e.g., less than about three hours or less than about one hour). The resulting superconductor layer can have a relatively low defect density and/or a relatively high critical current density.
Another potential advantage of the invention is that it can provide trifluoroacetate-containing compositions having a low water content. Treating these compositions to form a superconductor layer, or an intermediate of a superconductor layer, can take a relatively short period of time. The superconductor layers or superconductor intermediate layers prepared using these compositions can be of high quality.
The methods and compositions of the invention can be particularly advantageous when used to prepare a superconductor (e.g., an oxide superconductor), or an intermediate of such a superconductor, in the form of an object having a relatively large surface area, such as a wafer or a tape.
In certain embodiments, the methods and compositions of the invention can be particularly advantageous when used to prepare relatively thin oxide superconductor layers (e.g., layers having a thickness of less than about 0.5 micron, such as less than about 0.4 micron or less than about 0.3 micron).
In some embodiments, the superconductor material is preferably formed of YBCO (e.g., YBa2Cu3O7xe2x88x92x).
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice of the invention, suitable methods and materials are described below. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.